It is desirable to monitor engineering conditions of tires, such as wear, internal pressure and internal temperature in order to reduce tire costs and maximize vehicle efficiency. Of course, it is advantageous to perform such monitoring in large truck tires, which are expensive.
Prior art methods of monitoring large truck tires have included passive integrated circuits embedded in the body of the tire, or self-powered circuits which are positioned external to the tire. The passive integrated circuits rely on inductive magnetic coupling or capacitative coupling to energize the circuit, thus providing power to the circuit from a source remote from the tire. Self-powered circuits positioned external to the tire are exposed to damage from the environment such as weather, road hazards and even vandalism.
Recent engineering advances have permitted the installation of monitoring devices having active integrated circuits within tires. One such device is described in U.S. Pat. No. 5,562,787 to Koch et al. entitled "Method of Monitoring Conditions of Vehicle Tires", incorporated herein by reference, and assigned to the assignee of the present invention. These devices include an active circuit powered by a dedicated long life, miniature battery and at least one sensor for detecting, optionally storing and transmitting real time engineering conditions within the tire. Such devices are capable of being programmed to remain in an active, but dormant condition, but will switch automatically to an "awakened" condition in response to an external signal or a condition which exceeds preset limits.
One of the problems faced with such active devices is that they are delicate electronic devices that must operate in the harsh environment of a tire. Thus it is important to secure these devices in the tires to minimize the effect of the harsh tire environment on them, while still permitting them to be exposed to this environment to allow accurate monitoring of the engineering conditions for the life of the tire. These active devices have previously been mounted in tires by first encapsulating the device or power-containing circuit in a material which forms a rigid or semi-rigid encasement about the device, thereby inhibiting straining of the device as a result of applied stresses. Such materials have included non-foam compounds such as urethanes, epoxies, polyester-styrene resins, hard rubber compositions, and the like. The encapsulated device is then placed into a green rubber material which forms a housing or is placed in a green rubber pocket or pouch that becomes part of the tire. The encapsulated device is then permanently set within the rubber material during a subsequent vulcanizing operation. The green rubber material containing the encapsulated device may be assembled to the green tire and then vulcanized with the tire. Alternatively, the green patch containing the encapsulated device may be separately vulcanized and then affixed to a cured tire with a suitable adhesive. In either event, the delicate electronic components of the active tag are exposed to high vulcanizing temperatures which may adversely affect their performance or shorten their life. Additionally, certain components of the device, such as the pressure sensor or a thermistor must remain open to the pressurized tire cavity. The flow of rubber occurring during the vulcanization process can effect the openings to the tire cavity. As set forth in U.S. Pat. No. 5,562,787, the opening to the tire cavity can remain open during the curing operations by placement of a dowel into the opening.
While the methods and apparatus of U.S. Pat. No. 5,562,787 provide an acceptable method of assembling an active chip into a tire cavity, an improved method and apparatus which increases the life of the active chip in the tire is desirable.